Arginine methylation of proteins is mediated by the Protein-Arginine Methyl Transferase (PRMT) family of enzymes, which are important in signal transduction, transcription, RNA transport, and splicing. PRMTs are divided into two types defined by their activity. Type I PRMTs (including PRMT 1, 3, 4, and 6) are characterized by the formation of asymmetric dimethylated arginine residues. Type II PRMTs, which include only PRMT5, are defined by the formation of symmetric dimethylated arginine residues. Substrates for PRMT5 include myelin basic protein (MBP) and the spliceosomal D1 and D3 proteins. PRMT5 (also known as Skb1Hs/JBP1) exists as homooligomeric complexes, which includes a dimer and tetramer. Homooligomerization of PRMT5 has an important role in its ability to methylate the MBP protein. PRMT5 (aka JBP1 Janus Binding Protein1) is a member of a family of enzymes that deposit methyl groups on arginine residues of proteins, including both histone and non-histone substrates.